Wireline logging is a common operation in the oil industry whereby down-hole electrical tools may be conveyed on a wireline (also known as an “e-line”) to evaluate formation lithologies and fluid types in a variety of boreholes. In certain wells there is a risk of the wireline cable and/or logging tools becoming stuck in the open hole due to differential sticking or key-seating, for example.
Key-seating may occur when the wireline cable cuts a groove into the borehole wall. For instance, this can happen in deviated or directional wells where the wireline cable may exert considerable sideways pressure at the contact points with the borehole. Since the logging tool diameter is generally much bigger than the groove cut by the wireline cable, a keyseat can terminate normal ascent out of the borehole and potentially result in a fishing job or lost tools in hole.
Differential sticking may occur when there is an overbalance between hydrostatic and formation pressures in the borehole, the severity of which may be related to a number of issues, including: (1) the degree of overbalance and the presence of any depleted zones in the borehole; (2) the character and permeability of the formations bisected by the borehole; (3) the deviation of the borehole, since the sideways component of the tool weight adds to the sticking forces; (4) the drilling mud properties in the borehole, since the rapid formation of thick mud cakes can trap logging tools and the wireline cable against the borehole wall; and (5) the geometry of the toolstring being logged on wireline, since a long and large toolstring presents a larger cross sectional area and results in proportionally larger sticking forces. Additionally, during wireline formation sampling, the logging tools and wireline may remain stationary over permeable zones for a long period of time which also increases the likelihood of differential sticking.